I've copied several into a Fonts folder on my SD card, but they are not yet appearing on my device.

Note that "Fonts" should be "fonts". These should then show up under the "settings" menu, under key 7 with some ebook types and under menu-> settings> inside an ebook supported by CR3 (e.g. RTF and FB2). You can't add fonts for MOBI but you can choose between the pre-installed fonts.

Probably FB2 is your best option, although it isn't the easiest format to find converters for (BookDesigner works though).

I recommend switching to the OpenInkPot firmware, see 20090222 snapshot. For LIT under OpenInkPot I explode to OEB (using ConvertLIT GUI), make a zip of the entire directory and rename it .oebzip. This is then readable directly by FBReader under OpenInkPot. However, I have been doing this for couple of years and now Calibre is a viable alternative which offers the option of exporting to MOBI or ePub. Both of these will work well using FBReader under OpenInkPot. If you had not raised the issue of fonts, using Calibre to convert LIT to MOBI would have been a good option for the standard firmware too.